The present invention relates to a polyamide resin composition having improved weathering resistance. More particularly, it relates to a polyamide resin composition having improved weathering resistance which has high strength and modulus of elasticity and is capable of retaining gloss and minimized in surface roughening even if left in an outdoor exposure environment, and which is therefore best suited for manufacturing external trim parts of automobiles and railroad vehicles, parts of building materials and housing equipment, and the like. The present invention also relates to the molded articles using such a polyamide resin composition.
Since polyamide resins have excellent mechanical properties, moldability, chemical resistance and electrical properties, polyamide resins are popularly used for manufacturing automobile parts, machine parts, parts of electrical and electronic devices, parts relating to building materials and housing equipment, etc. Especially polyamide 6 is excellent in surpassing toughness and chemical resistance and used as an extrusion or injection molding material for monofilaments, films, sheets, etc., for manufacturing a variety of industrial articles.
Many attempts have been made for the improvement of heat resistance and mechanical properties of polyamide resins by incorporation of inorganic fillers, typically glass fiber, to realize weight reduction of the molded products by supersession of metal materials, yielding a remarkable economical effect. However, the extent of improvement of modulus of elasticity that can be achieved by the addition of inorganic fillers has a limitation. Also, there takes place a sharp reduction of modulus of elasticity in the temperature region exceeding the glass transition point of the inorganic fillers, which puts great restrictions on design flexibility in substitution of metal with plastic materials.
Various proposals have been made concerning metal substitution with resin materials. For example, Japanese Patent Application Laid-Open (KOKAI) Nos. 63-101420 and 63-105057 proposed polyamide resins comprising terephthalic acid and/or isophthalic acid and hexamethylenediamine, and Japanese Patent Application Laid-Open (KOKAI) No. 50-61449, etc., disclosed polyamide resins comprising adipic acid and methaxylylenediamine. These polyamide resins, however, are high in glass transition point, so that it is very difficult to obtain a molded article having excellent thermal properties and visual appearance, especially surface gloss, unless the mold temperature is set at a high level, not lower than 120° C. in injection molding.
Further, polyamide resins are very susceptible to oxidative degradation, and tend to incur undesirable phenomena such as lowering of mechanical strength, surface cracking, tinting, etc., with reduction of the degree of polymerization. It is notable that such oxidative degradation is accelerated by the external factors such as heat and light, so that use of the molded products of these polyamide resins in outdoor exposure environment is restricted.
Japanese Patent Application Laid-Open (KOKAI) No. 48-93652 and Japanese Patent Publication (KOKOKU) No. 63-29823 proposed to add various types of stabilizer in polyamide resin for preventing the said degradation phenomena. A typical means therefor, i.e. addition of an amine compound or a phenolic compound as an organic stabilizer in polyamide resin, is disclosed in Japanese Patent Publication (KOKOKU) Nos. 37-8183, 38-24725 and 36-2187.
However, the weathering resistance improving effect of these proposals is still unsatisfactory, and further improvement has been desired.
Japanese Patent Application Laid-Open (KOKAI) No. 8-259809 proposed a polyamide resin composition comprising an aromatic polyamide copolymer and an inorganic filler, which is excellent in moldability. However, There is no teaching for weathering resistance of the molded article therefrom. Further, there is a teaching that by blending polyamide 66 resin into the polyamide resin composition, the mechanical strength thereof is deteriorated because the water absorption become large. Therefore, in the above KOKAI, polyamide 66 resin is not blended into the polyamide resin composition from the above reason.
As a result of the present inventors' earnest studies to solve the above problems, it has been found that these problems can be eliminated by incorporating a specific aromatic polyamide resin into polyamide 6 and/or polyamide 66 resin.
The present invention has been attained on the basis of the above finding.